In many installations in the upstream or downstream oil and gas industry, chemical and petrochemical industry, it is required to introduce a mixture of liquid and gas into a processing vessel. The vessel can be a separation vessel designed to separate the stream, e.g. a natural gas stream comprising oil and/or water into liquid and gas streams. The vessel can also be a gas/liquid contacting vessel in which gas and liquid are counter-currently contacted to exchange heat or matter. An example of such a gas/liquid contacting vessel is a fractionation or distillation column, and a particular example is a vacuum distillation column.
In the specification and in the claims the word ‘gas’ is used to refer to gas and to vapour.
British patent specification No. 1 119 699 discloses a fluid inlet device for introducing a mixture of liquid and gas into a distillation column.
The known fluid inlet device comprises an inlet flow channel having an inlet end for receiving the mixture of liquid and gas, and a plurality of curved guiding vanes placed one behind the other along the inlet flow channel, wherein each vane comprises an intercepting part extending towards the inlet end of the inlet flow channel, and an outwardly directed deflecting part. The intercepting and deflecting parts of each vane are arranged in such a way that the vane intercepts and deflects part of the mixed feed stream, and is capable of effecting a separation between liquid and vapour by inertia and centrifugal force.
During normal operation a mixture of gas and liquid is supplied to the inlet nozzle of a column, which is in fluid communication with the inlet end of the inlet device. The vanes are curved so as to deflect the mixture outwardly. The change in the direction of flow causes a (pre-)separation of the mixture in that liquid is forced onto the concave surface of the vane thereby forming a liquid-rich stream on the concave surface and a gas-rich stream in the remainder of the outlet channel between two vanes. After the streams have left the outlet channel, the liquid-rich stream moves downwards in the column under the influence of gravity, whereas the gas-rich stream flows upwards in the column. In a specific embodiment of the known device a liquid catching channel is arranged at the trailing rim of the vane perpendicular to the main flow direction along the vane. This channel serves to discharge all of the liquid that is separated by the vane sideways, i.e. perpendicular to the main flow direction.
An important parameter of a separating flow inlet device is the total remaining liquid entrainment in the gas, i.e. the remaining liquid content of the gas flowing upwards. Part of the liquid content is due to the incomplete separation in the inlet device. Another part, however, is due to re-entrainment of liquid that had already been separated, which re-entrainment occurs generally in the region where gas and liquid streams leave the inlet device.
Re-entrainment is a general concern, including in distillation and separation applications, since it presents a larger liquid load to downstream equipment. Re-entrainment lowers the overall separation efficiency of the inlet device, since liquid that was already separated on the concave side of the vane, and that should ideally find its way towards the bottom of the vessel, is still carried upwardly with the gas.
Generally, re-entrainment is expected to increase at high velocities, which are e.g. the consequence of minimization of vessel size in view of cost and area occupied, e.g. on an offshore installation.
Although the known fluid inlet device has been successfully applied since its invention, it is desired to be able to operate with lower entrainment and therefore better overall separation efficiency than presently possible.